Video communication over lossy packet networks such as the Internet or wireless links is hampered by packet loss. Indeed, video quality severely degrades in presence of lost packets.
Furthermore, video communication over lossy packet networks is hampered by limited bandwidth and packet loss. In fact, video coders commonly use predictive coding schemes in order to reduce temporal correlation, and the main drawback of these schemes is that even a single packet loss may cause errors during the decoding process that propagate in time.
Intra coding may be used to limit the effect of errors, however the high bit rate required limits its use in many applications.
Another common way to increase the robustness of the coded stream and to reduce the length of error propagation is the use of the Forward Error Correction (FEC) codes. This last solution provides robustness to packet loss at expenses of coding efficiency. The main limitation of Forward Error Correction codes is the “threshold effect”: when the number of lost packets exceeds the correction capability of the code (e.g. the number of redundant packets for Reed-Solomon codes), the code is unable to correct the errors, thus causing quality degradation.
Layered or scalable approaches essentially prioritize data and thereby support intelligent discarding of the data (the enhancement data can be lost or discarded while still maintaining usable video), however the video can be completely lost if there is an error in the base layer.
Multiple Description (MD) Coding attempts to overcome this problem by coding a signal into multiple independent bit-streams such that any bit-stream can be used to decode a baseline signal. If one stream is lost, the other streams can still be decoded to produce usable video, and most importantly, the correctly received streams enable improved state recovery of the corrupted stream. It is possible to use information from the multiple streams to perform state recovery at the decoder. The main problem of Multiple Description is coding inefficiency. At the same rate, a single description produces higher quality than Multiple Description.
To overcome this limitation, some solutions based on Unbalanced Multiple Descriptions (UMD) have been investigated by researchers.
With unbalanced operation, the descriptions have different importance during the video reconstruction process. This way, it is simpler to control the amount of redundant data to add.
To achieve unbalanced operation one can adapt the quantization, the frame rate and the spatial resolution.
With unbalanced operation, different descriptions have different importance during the video reconstruction process. Many different systems to generate the Unbalanced Descriptions (UD) have been analyzed in the related prior art. The existing techniques are based on:                particular transformations,        energy considerations,        frame resealing operations, and        different quantization operations.        
Many prior art solutions have been designed to send the descriptions over different channels. Indeed, some existing solutions exploit path diversity either at the link layer using different antennas, or at the Internet Protocol (IP) level using multiple senders.
Introducing path diversity at the link layer can complicate the design of the video devices, requiring link-layer modifications to allow cross-layer optimizations. On the other side, having a plurality of senders can complicate the network topology.
Document WO-A-2004/046879 discloses an apparatus and method for generating multiple descriptions of compressed data. In the apparatus and method, transform coefficients are generated from input data and quantized. An energy distribution of the quantized transform coefficients is generated. Based on the energy distribution, the transform coefficients are grouped into layers. By entropy coding different number of layers, multiple descriptions of compressed data are generated.
Document JP-2002/198821 discloses a method and a device for processing signal for transmission in wireless communication system. A multiple-description coder generates many different descriptions in a prescribed portion of signals in a wireless communication system by using Multiple Description Scalar Quantization (MDSQ) or another type of multiple description coding. The different descriptions in the prescribed portion of the signals are arranged in a plurality of packets, so that at least the first description in the prescribed portion may be arranged in the first packet and the second description may be arranged in the second packet. Each packet is transmitted by using a frequency hopping modulator and the hopping rate of the modulator is selected or constituted based, at least partially, on the number of descriptions generated with respect to the different portions of the signals.
In U.S. Pat. No. 6,801,532 a process of sending packets of real-time information at a sender includes the steps of initially generating at the sender the packets of real-time information with a source rate greater than zero kilobits per second, and a time or path or combined time/path diversity rate, the amount of diversity initially being at least zero kilobits per second. The process sends the packets, thereby resulting in a Quality of Service (QoS), and optionally obtains at the sender a measure of the Quality of Service. Rate/diversity adaptation decision may be performed at receiver instead. Another step compares the Quality of Service with a threshold of acceptability, and when the Quality of Service is on an unacceptable side of said threshold increases the diversity rate and sends not only additional ones of the packets of real-time information but also sends diversity packets at the diversity rate as increased.
In U.S. Pat. No. 6,754,203 a method for communicating data over a packet switched network comprises dividing data into a plurality of frames, with each frame described by at least a first and a second parameter. The second parameter has a high correlation. The first parameter is placed in a first and a second description, while the second parameter is interleaved to the first and second descriptions. The first and second descriptions are packetized and communicated over the network. Upon reception, the first parameters for a frame sequence are extracted from one of the packets, while the interleaved second parameters are extracted from both the packets. If a packet is lost, the missing first parameter may be obtained from another packet, while the missing second parameter may be reconstructed using a second parameter from the other packet.
In U.S. Pat. No. 6,757,735 a method and system for streaming media data to a fixed client and/or a mobile client are disclosed, providing for encoding media data to be streamed to a client into a first multiple description bit-stream and into a second multiple description bit-stream. The method then determines the appropriate plurality of servers from a network of servers onto which the first and second multiple description bit-streams should be distributed. Then it is provided for distributing the first and second multiple description bit-streams to the appropriate plurality of servers positioned at intermediate nodes throughout a network such that a client is provided with access to the media data via a plurality of transmission paths.
In U.S. Pat. No. 6,460,153 is described method based on a projection onto convex sets (POCS) for consistent reconstruction of a signal from a subset of quantized coefficients received from an N.times.K over-complete transform. By choosing a frame operator F to be the concatenization of two or more K.times.K invertible transforms, the POCS projections are calculated in R.sup.K space using only the K.times.K transforms and their inverses, rather than the larger R.sup.N space using pseudo inverse transforms. Practical reconstructions are enabled based on, for example, wavelet, sub-band, or lapped transforms of an entire image. In one embodiment, unequal error protection for multiple description source coding is provided. In particular, given a bit-plane representation of the coefficients in an over-complete representation of the source, it is provided coding the most significant bits with the highest redundancy and the least significant bits with the lowest redundancy. In one embodiment, this is accomplished by varying the quantization step-size for the different coefficients. Then, the available received quantized coefficients are decoded using a method based on alternating projections onto convex sets.
In U.S. Pat. No. 6,215,787 is disclosed a signal data processing method using equal importance packetization. Processing of image data and other types of signal data is provided by representing the signal data in such a way that, when separated into packets, all packets are of approximately the same importance. As a result, if some of the packets are, for example, randomly lost in a lossy packet network, the resulting degradation in a reconstructed version of the signal is substantially uniform regardless of which packets are lost. A given image or other signal may be separated into packets using an energy equalization process, a signal whitening process, or other suitable technique.
The topics considered form the subject of extensive technical literature, as witnessed e.g. by the following technical papers:                “Unbalanced Quantized Multiple Description Video Transmission using Path Diversity”, Sila Ekmekci, Thomas Sikora, Technical University Berlin, in which it is disclosed an approach to Multiple Description based on the Multiple State Video Coding achieving a flexible unbalance rate of the two streams by varying the quantization step size while keeping the original frame rate constant. The total bit-rate RT is fixed which is to be allocated between the two streams. If the assigned bit-rates are not balanced there will be PSNR (peak signal to noise ratio) variations between neighbouring frames after reconstruction. It is attempted to find the optimal rate allocation while maximizing the average reconstructed frame PSNR and minimizing the PSNR variations given the total bit-rate RT and the packet loss probabilities p1 and p2 over the two paths. The reconstruction algorithm is also taken into account in the optimization process. Results presenting optimal system designs for balanced (equal packet loss probabilities) but also for unbalanced path conditions (different packet loss probabilities) are reported;        “Unbalanced Multiple Description Video Coding With Rate-Distortion Optimization”, David Comas, Raghavendra Singh, Antonio Ortega and Ferran Marques, deals with the problem of robust streaming of video data over best effort packet networks, such as the Internet, proposing multiple descriptions coding to protect the transmitted data against packet losses and delay, while also ensuring that the transmitted stream can be decoded with a standard video decoder, such as the H.263 decoder. The video data is encoded into a high resolution, i.e., high quality, video stream (description) using an encoder that produces an H.263 compliant stream. In addition, a low-resolution video stream (description) is also generated by duplicating the important information from the high-resolution video stream. This information includes the headers, the motion vectors and some of the discrete cosine transform (DCT) coefficients of the high resolution video stream. The remaining DCT coefficients are set to zero in the low-resolution video stream. Hence both video streams are independently decodable by a standard H.263 video decoder. However, only in case of a loss in the high-resolution video stream, the corresponding information from the low resolution video stream is decoded, else the received high resolution video stream is decoded. Thus the system is an example of an unbalanced MDC system where the low-resolution description is used only in case of losses in the high-resolution description. An optimization algorithm is disclosed which, given the probability of packet loss, allocates bits to the high resolution and low resolution descriptions, and selects the right number of coefficients to duplicate in the low resolution description, so as to minimize the expected distortion. The article refers to MD video coder using a similar rate allocation scheme however generating balanced descriptions;        “End-To-End Rate-Distortion Optimized Mode Selection For Multiple Description Video Coding”, Brian A. Heng, John G. Apostolopoulos, and Jae S. Lim, discloses use of Multiple Description video coding to reduce the detrimental effects caused by transmission over lossy packet networks. Each approach to MD coding consists of a tradeoff between compression efficiency and error resilience. How effectively each method achieves this tradeoff depends on the network conditions as well as on the characteristics of the video itself. This paper proposes an adaptive MD coding approach, which adjusts to these conditions through the use of adaptive MD mode selection. The encoder selects between MD coding modes in a rate-distortion optimized manner to most effectively trade-off compression efficiency for error resilience;        “A Novel Error-Concealment Algorithm for an Unbalanced Multiple Description Coding Architecture”, Marco Fumagalli, Rosa Lancini, and Stefano Tubaro, discloses, in order to increase the robustness of the coded stream and to reduce the length of error propagation, the use of a Multiple Description Coding technique, applying a novel sequence-based Error Concealment (EC) algorithm to an unbalanced MD video coding system that generates a High-Resolution (HR) and a Low-Resolution (LR) description. In order to recover a loss in the current frames the EC algorithm takes into account not only the spatial neighboring of the region to which correspond the data loss, but it looks also at what will happen in a significant number of future frames looking both the HR and the LR descriptions;        “Unbalanced Multiple Description Video Communication Using Path Diversity”, John G. Apostolopoulos and Susie J. Wee, combines MD video coding with a path diversity transmission system for packet networks such as the Internet, where different descriptions are explicitly transmitted through different network paths, to improve the effectiveness of MD coding over a packet network by increasing the likelihood that the loss probabilities for each description are independent. The available bandwidth in each path may be similar or different, resulting in the requirement of balanced or unbalanced operation, where the bit rate of each description may differ based on the available bandwidth along its path. The MD video communication system disclosed is effective in both balanced and unbalanced operation. Specifically, unbalanced MD streams are created by carefully adjusting the frame rate of each description, thereby achieving unbalanced rates of almost 2:1 while preserving MD effectiveness and error recovery capability.        
From the foregoing description of the current situation, it emerges that there exists the need to define solutions capable of dealing with multicast video transmissions in a more satisfactory way as compared to the solutions according to the known art described previously.